Lunar eclipses occur when our closest celestial neighbour in space, the Moon passes through the Earth’s shadow and can only occur at a full moon. So why don’t we see a lunar eclipse once a month at every full moon? The Moon’s orbit is tilted by 5 degrees to the Earth’s orbit around the Sun, so the Moon actually spends most of its time above or below the plane of Earth’s orbit. Thus, most full moons do not pass through the Earth’s shadow and no eclipse occurs. However, about two to four times a year some part of the Moon does pass through the shadow and an eclipse occurs.

The Earth’s shadow consists of two parts – the penumbra and the umbra. The penumbra is the outer part of the shadow and is where the Earth blocks part but not all of the Sun’s direct light from reaching the Moon. The umbra or inner part of the shadow is the area where Earth blocks all of the Sun’s direct light from reaching the Moon. About 35% of lunar eclipses are total eclipses and during this type of eclipse, the entire Moon passes through the umbra part of the shadow.

The Moon is still illuminated in the umbra due to indirect sunlight that has been refracted in, and emerges from, the Earth’s atmosphere. The dust and air in Earth’s atmosphere scatters the blue coloured light and allows red light to pass by. The remaining light is a deep red or coppery colour. So, if the air is clear we see a coppery coloured Moon. However if there is extra dust etc. in the atmosphere (maybe due to a volcanic eruption) the eclipsed Moon can turn darker shades of red, sometimes even completely black as was the case on October 4th, 1884. This was due to the eruption of Krakatoa. It is also interesting to consider that if the Earth had no atmosphere then the Moon would be completely black during totality.

Unlike solar eclipses, lunar eclipses are safe to watch with your eyes, you don’t need any special protective filters or even binoculars or a telescope, though these do help make the coppery colour stand out.